A rational approach to heterometallic cluster formation is described that uses incommensurate symmetry requirements at two different metals to control the stoichiometry of the assembly. Critical to this strategy is the proper design and synthesis of hybrid ligands with coordination sites selective toward each metal. The phosphino-catechol ligand 4-(diphenylphosphino)benzene-1,2-diol (H2L) possesses both hard catecholate and soft phosphine donor sites and serves such a role, using soft (C2-symmetric) and hard (C3-symmetric) metal centers. The ML3 catecholate complexes (M = FeIII, GaIII, TiIV, SnIV) have been prepared and characterized as C3-symmetry precursors for the stepwise assembly (aufbau) of heterometallic clusters. While the single-crystal X-ray structure of the Cs2 [TiL3] salt shows a C1 mer-configuration in the solid-state, room-temperature solution NMR data of this and related complexes are consistent with either exclusive formation of the C3-fac-isomer with all PPh2 donor sites syn to each other or facile fac/mer isomerization. Coordination of these [ML3]2- (M = TiIV, SnIV) metallaligands via their soft P donor sites to C2-symmetric PdBr2 units gives exclusively pentametallic [M2Pd3Br6L6]4- (M = Ti, Sn) clusters. These clusters have been fully characterized by spectral and X-ray structural data as C3h mesocates with Cs+ or protonated 1,4-diazabicyclo-[2.2.2]octane (DABCO·H+) cations incorporated into deep molecular clefts. Exclusive formation of this type of supramolecular species is sensitive to the nature of the counterions. Alkali cations such as K+, Rb+, and Cs+ give high-yield formation of the respective clusters while NEt3H+ and NMe4 + yield none of the desired products. Extension of the aufbau assembly to produce related [M2Pd3Cl6L6]4-, [M2Pd3I6L6]4-, and [M2Cr3(CO)12L6]4- (M = Ti, Sn) clusters has also been realized. In addition to this aufbau approach, self-assembly of several of these [M2Pd3Br6L6]4- clusters from all eleven components (two MIV, three PdBr2, six H2L) was also accomplished under appropriate reaction conditions.
ASJC Scopus subject areas
- Colloid and Surface Chemistry